1. Field of the Invention
The invention relates to a manufacturing method of a cast-iron friction member.
2. Related Art
A frictional surface of a cast-iron friction member that is used for a vehicular disc brake rotor, a brake drum, a sliding part of a machine tool or the like is required to have high corrosion resistance and high wear resistance. Therefore, the surface of the cast-iron friction member has been formed with a coating film or zinc phosphate coating. However, the coating film or coating is worn or delaminated over long time, so that it is difficult to secure a sufficient corrosion resistance and wear resistance.
Compared to the above, a technology of performing a nitrocarburizing treatment on the frictional surface of the cast-iron friction member has been studied and utilized. The nitrocarburizing treatment is a surface treatment of enabling interstitial diffusion of nitrogen and carbon at the same time at a treatment temperature of A1 transformation point (727° C.) or lower, typically 550° C. to 580° C. In the nitrocarburizing treatment, since a hard layer of a nitrogen compound is formed on the outermost surface, it is possible to improve the corrosion resistance and wear resistance of the cast-iron friction member. Also, since a phase transformation is not accompanied in the nitrocarburizing treatment, the strain is less caused, compared to a carburization treatment and the like. As the nitrocarburizing treatment, a salt-bath nitrocarburizing treatment, a gas nitrocarburizing treatment, a plasma nitriding treatment and the like may be exemplified.
For example, JP-A-H06-307471 and JP-A-2010-053926 disclose performing a nitrocarburizing treatment on a surface of a cast-iron disc brake rotor so as to form a Fe—C—N based compound layer on the surface by the salt-bath nitrocarburizing treatment, thereby improving the corrosion resistance and wear resistance. However, the salt-bath nitrocarburizing treatment has a problem in terms of safety and environmental burden because harmful cyanide is used. Also, the salt-bath nitrocarburizing treatment has a problem that a porous layer is apt to be formed on an outermost surface of the formed compound layer.
JP-B2-3303741 discloses a gas nitrocarburizing treatment in which nitrocarburizing treatment processes under nitriding atmosphere of 200 Torr or lower including ammonia gas at a treatment temperature of 450° C. to 560° C. are performed two times interposing a diffusion treatment process therebetween. In this method, the harmful cyanide is not used, and the compound layer to be formed on the surface of the cast-iron friction member can be made to be dense. Also, since it is possible to suppress a thickness of the compound layer from being excessively increased, it is possible to improve the surface durability.
However, when the cast-iron friction member having the nitrogen compound layer formed on the surface thereof by the above method is exposed to outside environments over a long period, red rust (Fe2O3) may be formed on the surface due to influences of temperature and humidity. Even when the surface of the vehicular disc brake rotor is subject to the nitrocarburizing treatment, if the surface is eroded by the red rust (Fe2O3), vibration may occur or a braking force may be lowered upon braking. Also, in recent years, an aluminum alloy wheel having a high design property has been much utilized. Regarding this, when the red rust (Fe2O3) is present on a surface of the rotor, it can be securely seen even from the outside with naked eyes, so that an outward appearance of a vehicle is deteriorated.
Regarding the above problems, it is efficient to further form an iron oxide layer having Fe3O4 as a main component, which is referred to as black rust, on the surface of the nitrogen compound layer obtained by the nitrocarburizing treatment. Since the iron oxide layer is uniform and dense, it can effectively prevent the red rust (Fe2O3) from being formed on the surface of the iron-cast friction member.
As disclosed in JP-A-H03-285058, the iron oxide layer is formed by a method referred to as homo-treatment in which a target product is exposed to vapor of 400° C. to 500° C. for 1 to 1.5 hours. However, it is difficult to set conditions of the homo-treatment. That is, when the temperature is too high or the treatment time is too long, the red rust (Fe2O3) may be formed. Therefore, it is very difficult to form a uniform and dense black iron oxide layer over the entire frictional surface of the iron-cast friction member.
As a method replacing the homo-treatment, JP-B-S53-000371 discloses a method of treating a steel material in a mixed gas of a nitriding gas and a carburizing gas at 500° C. to 600° C. for 0.5 to 3 hours, taking out the steel material from a furnace at the treatment temperature, holding the same in air for 60 to 120 seconds to thus form a Fe3O4 film and then cooling the steel material having the film formed thereon to a room temperature in oil. According to this method, it is possible to continuously perform the nitrocarburizing treatment, the iron oxide layer formation and the cooling treatment, so that it is possible to form the iron oxide layer of a good quality on the surface of the steel material.
Patent Document 1: JP-A-H06-307471
Patent Document 2: JP-A-2010-053926
Patent Document 3: JP-B2-3303741
Patent Document 4: JP-A-H03-285058
Patent Document 5: JP-B-S53-000371
However, according to the method disclosed in JP-B-S53-000371, after the nitrocarburizing treatment is performed, the target product is taken out from the furnace, as it is with being heated, the iron oxide layer is thus formed and then the target product is rapidly cooled (oil cooling). Thereby, a crack may be formed in the nitrogen compound layer or the iron oxide layer including Fe3O4.